specify the empty field_name param to cope with the adodb schema change.

It seems like adodb does not support the syntax to send DEFAULT values with an ALTER statement.
Whilst mssql keeps an existing default value if your alter'ing the size of the variable, mysql replaces the old
column definition with whatever is provided, and therefore drops the default.

git-svn-id: http://mantisbt.svn.sourceforge.net/svnroot/mantisbt/trunk@4469 <a class="text" href="/?p=mantisbt.git;a=object;h=f5dc347c">f5dc347c</a>-c33d-0410-90a0-b07cc1902cb9